Threaded joints for steel pipes, which are widely used for connecting steel pipes used in equipment of the oil-producing industries such as oil country tubular goods and riser pipes, are constituted by a pin, which is an element with male or external threads provided on the end of a first tubular member, and a box, which is an element with female or internal threads provided on the end of a second tubular member. Connection of a threaded joint is carried out by threadingly engaging the male threads and the female threads, which are both tapered threads.
Typically, the first tubular member is a pipe such as a steel pipe for use in oil country tubular goods and the second tubular member is a different member in the form of a coupling. This type of threaded joint for steel pipes is referred to as a coupling type. In a coupling-type joint, a pin is formed on both ends of the pipe, and a box is formed on both sides of the coupling. There are also integral threaded joints for steel pipes in which a pin is formed on the outer surface on one end of a pipe and a box is formed on the inner surface of the other end of the pipe. In this case, the first tubular member is a first pipe, and the second tubular member is a second pipe, and the two pipes are connected without a coupling.
Oil country tubular goods are usually connected to each other using standard threaded joints prescribed by API (American Petroleum Institute) standards. However, in recent years, as the environments in which excavation and production of crude oil and natural gas are carried out have become increasingly severe, special high-performance threaded joints referred to as premium joints are increasingly being used.
In a premium joint, both the pin and the box have, in addition to tapered threads which make it possible to fasten the pin and the box, a sealing surface provided on a peripheral surface in the vicinity of the threads and a shoulder surface which acts as a stopper during connection of the joint. Radial interference is provided between the sealing surfaces of the pin and the box. When the joint is tightened until the shoulder surfaces of the pin and the box abut each other, the sealing surfaces of both members intimately contact each other around the entire periphery of the joint so as to form a seal by metal-to-metal contact. The shoulder surfaces not only functions as stoppers at the time of tightening but also bear a compressive load acting on the joint.
FIGS. 7(A) and 7(B) show schematic explanatory views of a typical premium threaded joint for steel pipes of the coupling type in which FIG. 7(B) is an overall view and FIG. 7(A) is an enlarged view of a portion of the joint. As shown in FIGS. 7(A) and 7(B), this type of threaded joint for steel pipes has a pin 1 which is a male threaded element provided on the end portion of a pipe and a box 2 which is a corresponding female threaded element provided on both sides of a coupling. On its outer surface, the pin 1 has tapered male threads 11 and an unthreaded cylindrical tip portion referred to as a lip (referred to below as a lip portion) 12 adjacent to the male threads 11. The lip portion 12 has a sealing surface 13 on its outer periphery and a shoulder surface 14 on its end surface. The sealing surface 13 is a tapered surface (frustoconical surface) which gradually decreases in diameter towards the tip of the pin.
Correspondingly, the box 2 has, on its inner surface, tapered female threads 21, a sealing surface 23, and a shoulder surface 24 which can threadingly engage with or contact or abut against the tapered male threads 11, the metal sealing surface 13, and the shoulder surface 14, respectively, of the pin 1. As shown in the figure, a lip portion having a shoulder surface on its end surface is in most cases provided at the tip of the pin. A gap 31 between the pin and the box is formed at the end of the lip portion adjacent to the male threads for the purpose of collecting a liquid or semisolid lubricant which was applied to the surfaces of the pin and/or box in order to prevent galling and seeped out from the applied surface during tightening (make-up) of the joint due to intimate contact of the threads and the sealing surfaces of the pin and the box.
Formerly vertical wells were dominant. In such wells, a threaded joint for steel pipes had sufficient performance if it could withstand the tensile load of the weight of pipes connected to it and if it could prevent leakage of high pressure fluid passing through its interior. However, in recent years, as wells have become deeper, as sloping wells or horizontal wells having a well bore which is curved underground have increased in number, and as development of wells in severe environments such as in the deep sea or in polar regions has increased, a strong demand has developed for an increase in resistance to compression and improvements in sealing performance of threaded joints for steel pipes, particularly under internal and external pressures.
In order to improve the resistance to compression and sealing performance of a premium threaded joint under internal and external pressures, patent document WO 2004/109173 (hereinafter referred to as Patent Document 1) proposes a threaded joint for steel pipes which, as shown in FIG. 2, has an extended lip portion including a nose portion 16 between the sealing surface 13 and the shoulder surface 14 at the end of a pin 1. The nose portion 16 of the pin 1 provides a non-contacting region 18 in which the opposing surfaces of the pin 1 and the box 2 do not contact each other. On the other hand, the sealing surfaces 13 and 23 and the shoulder surfaces 14 and 24 of the pin and the box contact each other. By elongating the lip portion of the pin 1 so as to form a nose portion 16 having a non-contacting region 18 with a cylindrical outer surface which does not contact the box between the end surface and the sealing surface 13 of the pin, the wall thickness of the lip portion and accordingly the wall thickness of the shoulder surface and the sealing surface can be increased within the constraints of a limited pipe wall thickness, and the resistance to compression of a threaded joint for steel pipes and its ability to form a seal which is resistant to internal and external pressure can be markedly increased.
It is disclosed in Patent Document 1 that as shown in FIG. 4, the sealing surface of a box may be of the shape having a combination of a toroidal surface (R) (a surface formed by rotating an arc about the center of the joint axis) on the side of the sealing surface closer to the threaded portion and a tapered surface (TA) (a surface formed by rotating a straight line about the center of the joint axis) on the side of the sealing surface closer to the shoulder, while the sealing surface of a pin is formed from a tapered surface preferably with the same slope as the tapered surface TA of the box. This shape can increase the stability of contact pressure of the sealing surfaces of the pin and the box under a variety of operating conditions.
FIG. 5 is a schematic view showing different shapes of the sealing surface of a box which contacts a tapered shape of the sealing surface of a pin. FIG. 5(A) shows an example in which the sealing surface of a box is constituted solely by a tapered surface (TA), FIG. 5(B) shows an example in which it is constituted solely by a curved surface (R), and FIG. 5(C) shows an example in which it is constituted by a combination of a tapered surface (TA) and a curved surface (R). In each case, the pin is constituted solely by a tapered surface.
As shown in FIG. 5(A), when the box is a tapered sealing surface (TA) as in the sealing surface of the pin, contact between the sealing surfaces of the pin and the box is concentrated at the border 51 thereof closest to the threads. As shown in FIG. 5(B), when the sealing surface of the box is a curved surface (R) and that of the pin is a tapered surface, the area of contact between the sealing surfaces of the pin and the box decreases, and the stability of contact may worsen. As shown in FIG. 5(C), when the sealing surface of the box is a combined surface constituted by a tapered surface (TA) and a curved surface (R) and that of the pin is a tapered surface, as described in Patent Document 1, the stability of contact can be increased under a variety of conditions of use.